The present invention relates generally to a printer, such as a computer printer, and more particularly to an apparatus for electromagnetically operating the character keys of a printer.
In the presently known printers, the various characters are struck by actuation of adjacent striker mechanisms. Each mechanism operates a hammer which drives the key against a print support.
One electromagnetic striker apparatus includes an inductor or core member, forming a horseshoe-shaped magnetic circuit, an electromagnetic coil mounted on the core member and a magnetic strip armature. The armature, which substantially aligns with the free ends or pole pieces of the core member pivots about an axis perpendicular to the plane of the horseshoe-shaped core member. When the coil is excited by a current, the armature moves towards, or away from, the core member and thereby operates the strike hammer. It will be noted that, in this particular arrangement, the armature moves substantially perpendicularly to the lines of magnetic flux which pass through the armature.
As discussed in detail below, the presently known striker apparatus have numerous and significant shortcomings.
In particular, the propulsive force exerted on the armature at the moment the actuating current begins to flow in the coil is relatively weak. As a result, the flight time of the strike hammer, i.e., the time necessary to effect printing of a character, varies considerably. This variance exists between different striker mechanisms and strokes of the same mechanism. The magnitude of the variance depends upon frictional forces between fixed and moving parts of the apparatus.
The striking action obtained with the presently known electromagnetic striker mechanisms is, therefore, poor in quality and restricted in speed. That is, the variation in flight times causes character misalignment on the print support. Further, the mechanical inefficiency of the known apparatus substantially restricts the speed of operation of the hammer.